1. Field of the Invention
The present invention relates to display devices employing organic electroluminescence (EL) elements and manufacturing methods thereof, in particular, to a display device with luminance variation of an organic EL element reduced, and a manufacturing method thereof.
2. Description of the Related Art
Recent years have seen a rapidly increasing demand for flat-screen display devices such as a flat-screen liquid crystal display with light weight and low-power consumption, compared with conventional Cathode-Ray Tube displays. The liquid crystal display, however, has problems in viewing angle and responsiveness.
In order to overcome the problems, passive-matrix and active-matrix display devices have recently attracted attention. The passive-matrix and active-matrix display devices employ a self-luminous organic electroluminescence element (referred to as “organic EL element”, hereinafter) which provides a wide viewing angle and quick responsiveness. In particular, actively developed is the active-matrix display device which is advantageous in achieving high-definition and a larger screen.
A display device employing organic EL elements includes a display panel using the organic EL elements, and a driving circuit driving the organic EL elements. The display panel includes the organic EL elements each having: a first electrode including aluminum (Al); a second electrode including Indium Tin Oxide (ITO) and facing the first electrode; and a luminescent layer provided between the first and the second electrodes. The first electrode, the second electrode, and the luminescent layer are provided on a substrate, such as glass, in matrix. The driving circuit includes a thin-film transistor (TFT) separately driving each organic EL element.
Further, studies are conducted on (i) the bottom-emission display device taking the light emitted by the organic EL element outside via the substrate, and (ii) the top-emission display device taking the light at the second electrode side facing the substrate. In the bottom-emission display device utilizing the active matrix technique, however, the TFT of the driving circuit is formed on the substrate. This makes difficult to secure an enough opening ratio.
Concurrently, the top-emission technique is free from restriction, caused by the TFT transistor, in opening ratio. Compared with the bottom-emission technique, the top-emission technique makes possible improving use efficiency of the emitted light. Here, the top-emission technique involves taking the light outside via the second electrode formed on the top surface of the luminescent layer. This requires the second electrode to achieve high optical transparency as well as high conductivity. A typical transparent electrically-conductive material of which the second electrode is made is metallic oxide, such as the ITO; however, the metallic oxide is higher than a metallic material in resistivity. Thus, a larger area in display panel causes a greater difference of the second electrode in wire length among luminescent pixels, and develops a significant voltage reduction between an edge of the power supply unit and the center of the display panel. Depending on the voltage reduction, the luminance varies, and thus the center of the panel becomes dark. In other words, a problem arises in that the voltage varies depending on arrangement positions of the organic EL elements provided on the display panel, and the voltage variation cause deterioration in display quality.
In order to prevent the problem, there is an effective structure which enables to supply, for each pixel, the power from a low-resistance wire provided in the lower part to a transparent electrode provided in the upper part.
Disclosed in Patent Reference 1 (Japanese Unexamined Patent Application Publication No. 2002-318556), for example, is a display device shown in FIG. 16.
FIG. 16 is a cross-sectional view of a luminescent pixel included in a conventional display device in accordance with Patent Reference 1. Briefly described hereinafter is a display device 700 of Patent Reference 1 with reference to FIG. 16. According to FIG. 16, the display device 700 includes a first electrode 720 and an auxiliary wire 730 both provided on a same surface of a substrate 710. The first electrode 720 and the auxiliary wire 730 include a low-resistivity electrically-conductive material, and are separately provided each other using the photolithography technique. Then, provided on the first electrode 720 is a light modulating layer 750. Provided over the light modulation layer 750 is a second electrode 760 including a transparent electrically-conductive material. Further, a part of the second electrode 760 is connected to the auxiliary wire 730 in an opening part 745. Here, the opening part 745 is partially provided on a barrier 740.
Similarly, Patent Reference 2 (Japanese Unexamined Patent Application Publication No. 2003-303687) discloses an organic luminescent display device with (i) a first electrode and a second power supply line each provided in a discrete layer of a single glass substrate, and (ii) a second electrode and a second power supply line connected via a contact hole. This makes possible reducing wiring resistance caused by the second electrode to decrease variation found on the display surface.
The conventional display devices disclosed in Patent References 1 and 2, however, have the second electrode and an auxiliary wire directly connected together. Thus, an overcurrent, flowing through the second electrode and the auxiliary wire, could affect the display device including a driving circuit. In addition, the overcurrent also flows through a luminescent unit, which possibly affects the reliability and the life time of the luminescent unit. Here, a regular current required to illuminate luminescent units per one sub-pixel is as much as 3 μA to 5 μA. Compared with the regular current, the overcurrent is a pulsed current several tens to several hundreds of times as great as the regular current. The overcurrent occurs by: static electricity in manufacturing a display panel; a current caused by some sort of noise, from outside, found in a finished display device; and short circuit caused by other pixels.
Further, the conventional display devices disclosed in Patent References 1 and 2 have the second electrode and the auxiliary wire directly connected together via a connecting part. In order to realize the structure, all the layers involving luminescent operation, including an electron injection layer, an electron transport layer, a hole injection layer, a hole transport layer, and a luminescent layer, are required to be formed, avoiding coating the connecting unit. Realizing the structure via the vacuum evaporation technique, for example, requires a use of high-definition mask. The use of the high-definition mask, however, faces an alignment problem in manufacturing a large-screen and high-definition display device in high productivity.
Proposed in Patent Reference 3 is a structure to realize a connecting unit which eliminates the use of the above high-definition mask and keeps the overcurrent off.
FIG. 17 is a cross-sectional view of a luminescent pixel included in a conventional display device in accordance with Patent Reference 3 (Japanese Unexamined Patent Publication Application No. 2007-73499). A luminescent device 800 shown in FIG. 17 has at least one of a first buffer layer 850 and a second buffer layer 870 provided between an auxiliary wire 830 and a second electrode 880. The auxiliary wire 830 is separated from a first electrode 820 by a barrier 840 and formed on a substrate 810. The first buffer layer 850 and the second buffer layer 870 represent structural layers of a luminescent unit including a luminescent layer 860.
The first buffer layer 850: includes a combination of a metallic compound and an organic material; and is entirely p-doped, including the luminescent unit and the connecting unit. The second buffer layer 870: includes a combination of an electron-transporting substance and an electron-donating substance; and entirely n-doped, including the luminescent unit and the connecting unit.
This structure allows (i) the luminescent device 800 to have an auxiliary wire formed near each luminescent pixel, and (ii) the connecting unit to have appropriate conductivity by a carrier doped in the first buffer layer 850 and in the second buffer layer 870. This makes possible reducing luminance variation among luminescent elements due to the voltage reduction of the second electrode 880.
The first buffer layer 850 and the second buffer layer 870 included in the conventional luminescent device 800 shown in FIG. 17 are respectively p-doped and n-doped in advance. In order to realize the buffer layers with p-doped and n-doped, both of the layers need to be mixtures of a dopant and a transporting material in being formed in layer. A manufacturing process employing the co-evaporation technique involves forming the buffer layers including the above mixtures.